The invention relates to alloys of the 7000 series having high mechanical strength obtained by conventional metallurgy and to a process for obtaining them.
Alloys having high mechanical strength in this group, in particular those having a high content of alloying elements, are generally prepared either by powder metallurgy or by powder deposition--see for example the applicant's application FR-A-2640644. However, these processes are complex, necessitate special installations and consequently lead to expensive products.
The patent EP-A 0241193 (example 1) also discloses high strength alloys belonging to the 7000 group and having relatively high mechanical tensile characteristics but which have been solidified under pressure in the form of bars (diameter 75 mm.times.100 mm) which are immediately hot extruded without preliminary homogenisation. It should be noted that these conditions are quite unusual, probably necessitate a vertical extrusion press and that this practice is undoubtedly unfavourable with regard to the strength of the container acting as a casting mould. Furthermore, this method is complex and the alloys thus obtained attain a yield stress of at most 768.1 MPa.
The applicants have therefore attempted to obtain Al-based alloys having high mechanical strength but having sufficient ductility and being relatively inexpensive, by conventional metallurgy.
The term conventional metallurgy refers to a process in which a solid product is obtained as the result of a mean solidification rate between liquidus and solidus &lt;600.degree. C./min and for which the solidified product is cooled roughly to the ambient temperature (&lt;100.degree.) before being subjected to the shaping operations and/or subsequent heat treatments on other tools.
This could be the process for the gravity or pressure casting of wrought products issuing from ingots or bars obtained by semi-continuous casting, the continuous casting of strips between rollers, etc.
The U.S. Pat. No. 5,221,377 also discloses alloys of the 7000 group which have high characteristics and are obtained by conventional metallurgy. However, to obtain these high mechanical characteristics, it is necessary to subject them to a complex artificial ageing process in three stages.